SQL Joins – The Differences

In SQL Server you can find several joins. In this post I will try to explain the diffences between them, and how you can use them in your advantage.

Joins
There are several different types of joins. Some of them look different, but are actually the same. For example, a Left Join is the same as a Left Outer Join. This also counts for Right/Right Outer Joins. And if you ever encounter a statement with just Join in it, this is converted by SQL Server to an Inner Join.

(Inner) Join
An Inner Join takes the results from both tables in the join, and combines data that matches. All data that doesn’t match the ON clause, isn’t shown in the result set.

Left (Outer) Join
A Left Join takes the 2 tables used in the join, and takes all records from the left table (1st table in your join), and matches and adds data from the right (2nd table in your join). All data from the 2nd table that doesn’t match the ON clause is filtered out.

Right (Outer) Join
A Right Join takes the 2 tables used in the join, and takes all records from the right (2nd table in your join), and matches and adds data from the left (1st table in your join). All data from the 1st table that doesn’t match the ON clause is filtered out.

Full (Outer) Join
A Full Join takes the records from both tables, and matches all the data. The data that doesn’t match is also shown, except with NULL values in the columns that are missing on the other side.

Left (Outer) Join – Right Key Is NULL
With this Join (can be found in code samples, download below), you will get all the results that are not included in the Left Join.

Right (Outer) Join – Left Key Is NULL
With this statement (can be found in code samples, download below), you will get all the results that are not included in the Right Join.

Full (Outer) Join – Both Keys Are NULL
With this join you get all the data that isn’t included in the Full Join result set.

Cross Join
The Cross Join is a “special” Join. This Join type selects all the possible combinations it can. This sounds weird, but in the image below and in the code samples, it’s much clearer.


Code samples:
CreateSources.sql
SelectStatements.sql

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